First question – I would say the closer your training modality is to your intended endurance event, the better. So if you’re planning on doing some form of running/hiking event, then walking on the ground will be most beneficial (I assume your “rolling track” option is on the ground?)
I’ll let others chime in re: AeT drift, your #2 option would be my best guess.
Also, re: “my HR drifts quite a lot…”, I wonder if you may be starting too high? Have you done an AeT test? (I assume you have considering you seem pretty specific with you AeT & AnT #s but just thought I’d 2x check)
In case you haven’t done one yet…https://www.uphillathlete.com/8-diy-steps-to-figure-out-your-aerobic-threshold-indoor/